18 ON SHOOTING STARS. 



Herrick estimated that about three times as many were to be seen in the morning as in the 

 evening. Mr. Coulvier-Gravier gives the following table"" for the moan hourly numbers at 

 different hours of the night. 



5" to C"— 7.2 12" to 1"— 10.7 



6 to 7— G.5 • 1 to 2—13.1 



7 to 8—7.0 2 to 3—16.8 



8 to 9—6.3 3 to 4—15.6 



9 to 10—7.9 4 to 5—13.8 



10 to 11—8.0 '" 5 to 6—13.7 



11 to 12—9.5 to 7—13.0 



* 



As the observations from which these numbers have been obtained have never been published, 

 we cannot say what confidence is to be given to them. 



DISTRIBUTION OF THE ORBITS OF THE METEOROIDS IN THE SOLAR SYSTEM. 



There are at least three suppositions respecting the distribution of the orbits of the 

 meteoroids in the solar system which are naturally suggested. Either of them may be con- 

 sidered as plausible, and one does not necessarily exclude another. 



1st. They may form a number of rings like the August group, cutting or passing near 

 the earth's orbit at many points along its circuit. The sporadic shooting stars may be out- 

 lying members of such rings. 



2d. They may form a disk in or near the plane of the orbits of the planets. 



3d. They may be distributed at random, like the orbits of comets. 



According to the first of these suppositions there should be a succession of radiants cor- 

 responding to the several rings. Professor Heis and R. P. Grey, esq., believe that they 

 have detected such a series. Continued observation directed to this end will probably decide 

 whether the meteoroids belong entirely, or mostly, to rings. 



According to the second supposition the apparent paths of the sporadic meteors should, 

 if produced in the direction of the motion, all cut the ecliptic below the horizon. For the 

 absolute motion of the meteoroid and the earth's motion being both in or near the ecliptic, 

 the resultant relative motion should be from some point in or near the ecliptic. That point 

 should be above the horizon, since the body must move downward to enter the atmosphere. 

 As the apparent path produced backward cuts this point, it will, if produced forward, cut the 

 ecliptic below the horizon, t We have thus a simple means of determining whether the 

 sporadic meteors come exclusively, or even largely, from a disk or lenticular-shaped group 

 like that which the Zodiacal Light is often supposed to indicate. 



If the third supposition is true, the mean velocity of the meteors is a function of the 

 numbers of shooting stars in the different hours of the night. For if the velocity is very 

 small few would be seen in the evening, while if it is very large there should be nearly as 

 many in the evening as in the morning. 



Recherehes sur les Meteores, p. 2 10. 



J The earth's attraction tends to turn the direction of the path downward, or carry the point from which the meteor comes 

 away from the ecliptic towards the zenith. This will sometimes', though rarely, affect the above rule. 



